Lets start with some of my favourite essential resources

R Markdown: The Definitive Guide by Yihui Xie, J. J. Allaire, Garrett Grolemund.
R Markdown Cookbook by Yihui Xie and Christophe Dervieux
Pimp by RMD: a few tips for R Markdown by Yan Holtz.
Reference Guide
R Markdown Cheat Sheet
R Markdown formats - R for data science
RMarkdown overview/references

If are a R Markdown newbie I suggest starting at the offical lessons on R Markdown by R Studio.

Here is a presentation I have presented in the past to give you a give overview of using RMarkdown

INSTALL Just like the rest of R packages, get started with R Markdown by executing the following from the console

install.packages("rmarkdown")

Create a new RMarkdown document by selecting File > New File > R Markdown... > Document > HTML > Open This will open the default RMarkdown document that will knit to html, however you can change to PDF, word etc.

Templates

To get the most out of RMarkdown there are a number of various templates to suit whatever document and style you need. Below are some links to get you started.

General links on RMarkdown templates